A semiconductor device in which elements formed on a semiconductor substrate are isolated from one another by an isolation region for the purpose of realizing high integration of an integrated circuit device is known as a conventional semiconductor device. Also, a semiconductor device, in which an impurity layer is provided under isolation region to connect between wells having the same conductivity type with a low electric resistance thereby obtaining a common potential to the wells, is known as the semiconductor device having such isolation regions formed therein. This technique, for example, is described in Japanese Patent KOKAI No. H09-213786.